The present invention relates to a fuel injector, and to a method for operating a fuel injector.
A fuel injector for fuel-injection systems of internal combustion engines is known from German Patent Application No. 195 38 791, where a valve-closure member that interacts with a valve-seat surface to form a sealing seat is controlled by an actuator, via a valve needle.
The principal problem in using piezoelectric actuators is their thermal expansion. In contrast to customary materials such as steel or plastics, the piezoelectric materials have a negative coefficient of thermal expansion. This causes the piezoelectric actuator to contract with increasing temperature, while the surrounding housing expands. The different thermal expansion coefficients of the piezoelectric actuator, on one hand, and the housing, on the other hand, produces a temperature-dependent valve lift when this is not compensated for by appropriate measures.
Temperature compensation for a first piezoelectric actuator by a second piezoelectric actuator is known from the dissertation of Niko Herakovic, xe2x80x9cDie Untersuchung der Nutzung des Piezoeffektes zur Ansteuerung fluidtechnischer Ventilexe2x80x9d [xe2x80x9cAnalyzing the Use of the Piezoelectric Effect for Controlling Fluid Valvesxe2x80x9d], TH Aachen 1996, pp. 75-77, Wissenschaftsverlag [Scientific Publishing House] Aachen, ISBN 3-89653-041-0. In this case, the two piezoelectric actuators are each accommodated in one housing. To compensate for temperature, the second piezoelectric actuator counteracts the first piezoelectric actuator on a cylinder disposed between the two piezoelectric actuators. The cylinder is raised as a function of the operating voltage of the first actuator. When the temperature of the two actuators is increased, then the thermal expansions of the two actuators compensate for each other.
A disadvantage of the temperature compensation known from this printed publication is that, in order to actuate a valve needle of the fuel injector, the valve needle must be connected, via a suitable connecting device, to the cylinder supported between the two actuators. Additional parts encompassing at least one of the actuators are necessary for this, which means that the width of the fuel injector increases. In addition, the actuators are spaced far apart from each other so that, in response to the first piezoelectric actuator warming up sharply as a result of operating conditions, the second actuator is not able to compensate for the thermal expansion of the first actuator. Even in long-term operation, the temperature gradient formed between the first piezoelectric actuator and the second piezoelectric actuator results in insufficient temperature compensation. In the exemplary embodiment of the dissertation, the temperature of the two actuators is actively adjusted by cooling or heating elements. In summary, this temperature compensation is costly and not suitable for practical use.
To compensate for temperature, German Patent Application No. 195 38 791 proposes designing the valve housing as two pieces made of different materials. For example, it is proposed that one housing part be manufactured from steel, and the other housing part be manufactured from Invar. By choosing a suitable length for the first housing part made of steel and the second housing part made of Invar, it is intended that the total, resulting thermal expansion of the housing be adapted to the thermal expansion of the piezoelectric actuator, and therefore, that the piezoelectric actuator and the housing encompassing the piezoelectric actuator expand in the same manner, as a function of temperature.
A disadvantage of this design approach is that the valve housing is difficult to manufacture, and the material for the second housing part, preferably Invar, is expensive. Furthermore, it must be taken into consideration, that the valve housing and the actuator can have different temperatures. Thus, the waste heat of the piezoelectric actuator, which especially results from frequent operation of the fuel injector, can heat up the piezoelectric actuator, and it can only transfer its temperature slowly to the valve housing. On the other hand, the temperature of the valve housing is affected by the waste heat of the internal combustion engine on which the fuel injector is mounted. Therefore, this type of temperature compensation is not satisfactory.
A fuel injector for fuel-injection systems of internal combustion engines is known from German Patent No. 195 19 192, where an actuator acts on a valve needle, via a hydraulic transmission system. The transmission device includes a primary piston having an inner opening, in which a secondary piston is moveably guided. The secondary piston is connected to a valve needle, which is sealingly and movably guided in the valve housing. In the valve housing is a working chamber, which is filled with fuel and delimited by the primary piston and the secondary piston. The piezoelectric actuator contacts the primary piston on the side of the primary piston opposite to the working chamber. Since the volume of the fuel-filled working chamber must be maintained, a movement of the primary piston due to the action of the piezoelectric actuator causes the secondary piston to move in the primary piston, a suitable stroke transmission ratio being given by appropriately dimensioning the surfaces on the primary piston and the secondary piston, on the side of the working chamber. The temperature compensation is attained through a defined slot between the primary piston and the secondary piston. To that end, a portion of the fuel can be expelled from the working chamber in response to a temperature-dependent, quasistatic expansion of the fuel in the working chamber.
A disadvantage of this design approach is that the hydraulic temperature compensation causes the action of the actuator to be transmitted to the valve needle in a damped manner, which increases the response time of the valve needle, and does not allow the fuel injector to be used as a rapid-actuation fuel injector.
In contrast, the fuel injector of the present invention has the advantage that the temperature compensation of the actuator is considerably better. In addition, the fuel injector according to the present invention can also be used as a rapid-actuation fuel injector. Further advantages lie in the precise adjustability of the course of injection, whereby the injection operation can be adjusted to the specific operating condition and the operating requirements of the internal combustion engine; and in the small number of mechanically movable components, so that the fuel injector is designed to have a low rate of wear, and is easy to construct.
It is advantageous when the supporting element lies against a shoulder formed in the valve housing. This can reduce the number of additional parts. In this case, the supporting element can rest against a shoulder formed in the valve housing, by means of an elastically deformable seat element. This allows the valve needle to sit centered in the sealing seat. In addition, abrupt pressure pulses acting on the valve needle can be absorbed, which means that stress on the valve needle is reduced.
It is also advantageous when a large, initial stress is applied to at least one of the actuators, which means that, in the case of non-actuated actuators, the valve needle is supported against the sealing seat, in the closed position, by a force given by the difference in initial stress. This can eliminate the need for an additional compression spring for pressing the valve needle into the sealing seat.
The supporting element is advantageously secured in the valve housing by a screw element, which allows the initial stress acting on at least one of the actuators to be adjusted. This allows the contact force of the valve needle in the sealing seat, and the opening force acting on the valve needle in the case of non-actuated actuators, to be discreetly adjusted. This is especially useful in connection with the elastically deformable seat element. This also allows the ratio of the initial stresses of the two actuators to be set.
The actuators are advantageously arranged in an oblong actuator housing, the actuator housing having at least one notch, which is laterally positioned on the actuator housing, and has an elongated design in the longitudinal direction of the actuator housing; the supporting element protruding through the notch, and being movable in the notch, in the longitudinal direction of the actuator housing. The actuator housing allows the two actuators to be prestressed, which has a favorable effect on the operational reliability of the fuel injector, since unfavorable tensile forces on the actuators are avoided. In addition, the actuators can be preassembled in the housing in a favorable manner. Using the notch having an elongated design, the actuator housing can also be guided through the supporting element in the valve housing.
It is also advantageous when the actuator housing includes a housing plate on the inflow side, a housing plate on the side of the sealing seat, and a tubular housing wall having the oblongly designed notch; at least one of the actuators acting on the valve needle, via at least one of the housing plates. This allows the actuator housing to be installed in the fuel injector in a compact manner, which results in a favorable transfer of force to the valve needle.
In response to a change in temperature, the actuator arranged on the one side of the supporting element advantageously undergoes an expansion, which is in the direction of the supporting element and compensates for an expansion of the actuator arranged on the other side of the supporting element, the latter expansion being in the direction of the supporting element, and being produced in response to the same temperature change. This attains a particularly effective temperature compensation.
The method of the present invention for operating a fuel injector, has the advantage that the closing and opening of the sealing seat can be actively controlled in both directions, without requiring additional components.
The valve needle advantageously closes in response to the second electrical operating voltage of the second actuator being switched off. This allows all of the energy used to actuate the first actuator to be used for closing the sealing seat, whereby the closing operation is simplified.
The sealing seat is advantageously opened up to a first opening cross-section by switching off the first operating voltage of the first actuator, while the second operating voltage of the second actuator is switched off. The sealing seat is opened up to a second opening cross-section by applying an electrical operating voltage to the second actuator, while the first operating voltage of the first actuator is switched off. This achieves a larger, two-stage valve-needle lift without the requirement of additional components.